Transcriber's note: Text enclosed by underscores is in italics (_italics_). A single underscore NH_3 introduces a subscript, and a caret 30^a a superscript. Rapid in all directions, scientific progress has during the last generation been more rapid in the direction of Biology than in any other; and had this work been one dealing with Biology at large, the hope of bringing it up to date could not have been rationally entertained. But it is a work on the _Principles_ of Biology; and to bring an exposition of these up to date, seemed not impossible with such small remnant of energy as is left me. Slowly, and often interrupted by ill-health, I have in the course of the last two year…
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